Packaging materials used for the packages of foods, medical and pharmaceutical products, and the like are desired to prevent the degradation of contents. For example, packaging materials for foods are desired to minimize the oxidation and degradation of proteins, fats and oils, and the like and to keep flavor and freshness. Moreover, packaging materials for medical and pharmaceutical products required to be handled in aseptic conditions are desired to minimize the degradation of the active ingredients of contents and to keep the effects. Such degradation of contents is mainly caused by oxygen and water vapor passed through packaging materials or other gasses reacting with the contents. Therefore, packaging materials used for packaging foods, medical and pharmaceutical products, and the like are desired to have properties (gas-barrier properties) that prevent passage of gases such as oxygen and water vapor.
To attempt to meet these demands, a gas barrier film formed of a polymer (gas-barrier polymer) that has relatively high gas-barrier properties, and a laminate using this film as a substrate film are conventionally used.
Conventionally, such a polymer is used for a gas-barrier polymer in which a high hydrogen bonding group having high hydrophilic properties is contained in a molecule, as typified by poly(meta)acrylic acid and polyvinyl alcohol. However, although packaging materials formed of these polymers have highly excellent gas-barrier properties against oxygen and the like under dry conditions, the packaging materials have a problem in that under highly humid conditions, the gas-barrier properties against oxygen and the like are greatly degraded because of the hydrophilic properties of the packaging materials, or have a problem in that the above film has poor resistances against humidity and hot water.
To solve these problems, it is known that a polycarboxylic acid based polymer layer and a polyvalent metal compound containing layer are adjacently deposited on a substrate and a polyvalent metal salt of a polycarboxylic acid based polymer is formed by an intercalation reaction (as described in PTLs 1 and 2, for example). According to the disclosures, gas-barrier packaging materials thus obtained have high gas-barrier properties against oxygen under high humidity.
Moreover, there is known a gas-barrier packaging material that does not rely on humidity and does not need retort processing and the like (as described in PTLs 3 to 5, for example)